"Ventilators" are medical devices for delivering a breathing gas to a patient. Typically, variables such as the patient's breathing rate or frequency, volume of breathing, and inspiratory flow rate may be controlled by the ventilator operator. Usually, ventilators employed in hospital critical care units provide a supply of air enriched with oxygen for inspiration by the patient, and may conventionally include controls for either assisting or controlling breathing, exhaled volume indicators, alarms systems, positive end expiratory pressure valves, pressure indicators, gas concentration monitors, flow indicators, and heated humidifiers for warming and humidifying the breathing gas. Ventilators intended for use with anesthetized patients are usually much simpler to operate than ventilators intended for use in critical care units, but require the anesthetist to add specific ancillary devices and accessories to the "patient breathing circuit" connecting the ventilator to the patient, as warranted by factors such as the physiological status of the patient, the nature of the surgical procedure, the anesthetic technique employed, etc.
Typically, in both anesthetic and critical care applications, the "patient breathing circuit" includes hoses which connect the ventilator and ancillary devices and accessories to an endotracheal tube inserted into the patient's trachea to permit breathing gas to pass through the trachea into the patient's lungs.
The following discussion pertains to ventilators and patient breathing circuits intended for anesthetic applications, since it is believed that these are the more demanding and crucial applications, but it should be understood that the discussion applies generally to applications in which ventilators are employed in critical care units.
It sometimes happens that the tubing used to convey breathing gas to the anesthetized patient becomes disconnected, or blocked, or develops leaks, or that the endotracheal tube becomes dislodged; any of which may result in an interruption in the supply of breathing gas to the patient. Interruption of the patient's breathing gas supply for a relatively brief period may have serious consequences including hypoxia, cardiac arrest or even death. Since an anesthetized patient is often intentionally paralyzed with curare-like drugs, and since the attending anesthetist's view of the patient may be largely obscured by the surgical drapes, the positioning of the patient, the nature of the surgical procedures, other equipment, etc., it is often not possible for the anesthetist to observe the patient and patient breathing circuit to visually detect hazardous conditions. Recent studies have shown that even very experienced anesthetists may not be able to recognize symptoms of an interruption in the patient's supply of breathing gas until after the interruption has gone undetected for a period of time sufficient to result in hypoxia, bradycardia or cardiac arrest.
"Disconnect monitors" (also known in the art as "patient-circuit monitors", "breathing-circuit monitors" or "low pressure alarms", but hereinafter collectively referred to as "disconnect monitors") are devices for monitoring a patient's breathing gas supply and for producing alarm signals upon detection of symptoms of a disconnection or blockage in the patient breathing circuit. Prior art disconnect monitors have utilized pressure sensors, gas volume sensors or gas flow sensors to monitor the pressure, volume or flow characteristics of the breathing gas supplied to a patient. Typically, such prior art disconnect monitors produce alarm signals upon detection of breathing gas pressures, volumes or flow rates which fail to meet operator defined criteria. However, such prior art disconnect monitors have proved incapable of reliably detecting certain types of disconnections or blockages in the patient breathing circuit. For example, the tubing which supplies breathing gas to the patient may become disconnected, and the open end of the tubing may be partially or totally blocked if it rests against a surgical drape, pillow or bedsheet. Such partial or total blockage of the tubing has been found to result in a back pressure within the tubing sufficient to maintain the breathing gas pressure in the vicinity of a disconnect monitor pressure sensor within acceptable limits. Accordingly, some prior art disconnect monitors have failed to produce an alarm signal even though the patient's breathing gas supply from the patient breathing circuit is completely cut off.
Some prior art disconnect monitors have also proved incapable of reliably detecting interruptions in the patient's breathing gas supply caused by partial extubation of the endotracheal tube or kinking of the breathing gas tubing, either of which may also result in back pressures sufficient to maintain the breathing gas pressure in the vicinity of the disconnect monitor pressure sensor within acceptable limits and thereby inhibit production of an alarm signal even though the patient's breathing gas supply is partially or completely cut off.